This invention relates to a process for producing a polytetrafluoroethylene porous film, and more particularly to a process for producing a porous film having excellent strength, and substantially circular and relatively uniform pores wherein the porous film is obtained using, as starting materials, polytetrafluoroethylene molding powders obtained by a suspension polymerization method.
Polytetrafluoroethylena resins (hereinafter sometimes referred to as PTFE) have excellent chemical resistance, heat resistance and mechanical characteristics, and therefore have been used in a variety of fields. For example, porous films comprising PTFE have been widely used as a filter for corrosive materials or for high temperature materials because of the characteristics as described above. Further, the porous films comprising PTFE have been also used as electrolysis diaphragms, fuel cells and the like.
Heretofore, porous films have been produced from PTFE resins by a process wherein a liquid lubricant is incorporated into PTFE fine particles designated as fine powders of average particle size of 0.1-0.4 micrometer obtained by emulsion polymerization of tetrafluoroethylene; the blend is compression molded to prepare a preform; it is then formed into a film by processes including extrusion, rolling or combination; the liquid lubricant is then removed; and thereafter the resulting PTFE film is uniaxially or biaxially stretched under conventional heating conditions. For example, Japanese Patent Publication No. 42794/1978 discloses a process for producing a PTFE porous article which comprises the steps of heating a sintered PTFE film to a temperature of at least 327.degree. C., thereafter slowly cooling it, heat-treating such that its crystallinity is at least 80%, and thereafter uniaxially stretching it at a stretch ratio of from 1.5 to 4 at a temperature of from 25.degree. to 260.degree. C.
However, it is difficult to produce substantially perfect circular pores having a predetermined size in the PTFE porous films produced by such processes. Further, the mechanical strength of the porous film is not entirely sufficient.
We have carried out studies in order to overcome the problems described above and found the following facts to achieve the present invention. That is to say, (a) in producing the PTFE porous films, it is preferable to use, as starting materials, PTFE molding powders obtained by suspension polymerization rather than PTFE fine particles designated as the above fine powders obtained by emulsion polymerization; and (b) the pore characteristics and mechanical strength of the resulting PTFE porous films are greatly improved by preheating a film-shaped PTFE to a temperature of at least 327.degree. C. and thereafter quenching it at a cooling rate of at least 70.degree. C. per hour to reduce the crystallinity thereof to 55% or below prior to stretching the film-shaped PTFE under heating conditions .